Bogged Down

Q. I’ve been reading your articles for many years and truly love the wealth of knowledge it provides your readers. I’ve recently been having a small but irritating problem with my ’94 GMC 1500 pickup, which I love and have had for over 12 years. It has a TBI 5.0L engine with 166,000 miles. It’s extremely reliable, but under full acceleration it bogs and then accelerates. I’ve measured fuel psi and timing, and can’t seam to resolve the problem. Thanks for any help you can provide.

Steve Guerrette
Via email

A. The sheer volume of cars and trucks produced with throttle-body injection systems is staggering. I can’t even begin to guess how many cars and trucks are running around with TBIs. They have been used on big-blocks, small-blocks, V-6s, and even four-cylinders. They used this technology from 1980 in GM’s first front-wheel-drive passenger car, the Citation with the Iron Duke four-cylinder, to 1995. So you can imagine the numbers.

TBI systems are a speed density system, the most common system today is a Mass Air Flow system. As the speed density system works off of a fixed volumetric efficiency fuel table, the mass airflow system works off of an airflow sensor in the intake air stream. This system fuels the engine based on the air ingested into the engine. It can correct the air/fuel ratio based on this increased/decreased air intake and accept part changes and continue to fuel the engine correctly.

Back to your bog on acceleration: Does the engine bog when you tip into the throttle, or after tip-in and it falls flat and recovers? You say you checked the fuel pressure; did you check it while the engine was bogging, or just static in the shop stall? If the engine bogs on throttle tip-in, look to the throttle position sensor. This is an electrical rheostat connected to the throttle shaft of the TBI unit. It tells the computer where the throttle blade is and how quickly you’re opening the throttle. There is a table in the fuel injection calibration that is called “accelerator enrichment”. This is a time-based table that injects more fuel the quicker you open the throttle—in other words, an accelerator pump. The throttle position sensor over time wears out in the idle and off-idle positions, giving the computer bad data.

Next, look at the O2 sensor. This is a sensor in the exhaust stream that monitors the burned fuel in the exhaust stream. It’s truly the lack of oxygen in the exhaust. This sensor trims the air/fuel table based on how well the engine is running and keeping the air/fuel ratio near the 14.7:1 target. When these sensors age they can trim the fuel tables to the lean side, which can also lead to a bog.

Finally, let’s go back to your fuel pressure test. The fuel filters on GM EFI (all systems), are woefully inadequate, and should be replaced frequently. The undersized fuel filter on GM trucks and cars is the main reason that so many fuel pumps fail in the field. The filters become restricted and the pumps have to work extra hard to keep up with demand. Your fuel pressure test will come up good at idle in a stall with 12 psi. As soon as you tip into the throttle and increase the demand for fuel, the pump can’t keep up pushing through a plugged filter. If you haven’t changed your fuel filter it should be the first thing you do. If it doesn’t help with your bog issue it will certainly increase the life of your fuel pump. Good luck with your bog diagnostics. Hopefully we’ve put you on the right track.